1. Technical Field
This invention relates to spectrum projectors. More particularly, this invention relates to variable acousto-optic spectrum projectors where the intensity of different wavelengths of the projected radiation is controlled as a function of time.
2. State of the Art
All objects of non-zero absolute temperature emit thermal radiation. Spectral energy density f(.lambda., T) is given by Planck's radiation law: ##EQU1##
which is strictly valid for a blackbody, where h=6.626.times.10.sup.-34 J s (Planck's constant), c=2.998.times.10.sup.8 m/s (speed of light), .lambda. is wavelength, k=1.381.times.10.sup.-23 J/K (Boltzman constant), and T is absolute temperature. For bodies at room temperature (T=300 K), this yields a spectrum with a maximum intensity at approximately 10 .mu.m wavelength in the middle infrared spectral range. If the temperature is increased, the spectral energy distribution will vary according to EQ. (1), and the wavelength at maximum intensity (.lambda..sub.max) will be displaced towards shorter wavelengths. For T=6000 K, the temperature of the surface of the sun, .lambda..sub.max is in the visible range. This displacement of .lambda..sub.max as a function of temperature is approximated by Wein's displacement law: EQU .lambda..sub.max.multidot.T=constant=2.898.times.10.sup.-3 m.multidot.s, (2)
which can be derived from EQ. (1). By integration over all radiation frequencies, one derives Stefan-Boltzmann's radiation law: EQU R=.sigma.T.sup.4, (3)
where the total emittance, R, is the total energy of all wavelengths emitted per unit time and per unit area of the blackbody, T is the kelvin temperature, and a is the Stefan-Boltzmann constant, equal to 5.672.times.10.sup.-8 W/m.sup.2 K.sup.4. It should be noted that the total emittance for an outside surface of a body of an object is always somewhat less than R in EQ. (3), and is different for different materials. A good approximation of total emittance for non-blackbody objects is: EQU R=.epsilon..sigma.T.sup.4, (4)
where .epsilon.&lt;1, and is termed the body's emissivity.
Electromagnetic radiation sources are used in products ranging from lights to X-ray machines. For example, in a conventional infrared spectrometer one will typically find a hot radiation source, an optical filter that selects a restricted spectral region from the continuum of radiation emitted by the source, a chamber containing a sample which is radiated, and a detector that measures radiation passed through the sample. Usually, the radiation sources of such spectrometers operate at a constant temperature T.sub.h, which is much higher than the background, or ambient, temperature, T.sub.o.
For many practical instruments it is useful to modulate the emitted radiation either spectrally, temporally, or both. One conventional method of creating pulsed radiation is to insert a rotating wheel (a chopper) furnished with equidistant apertures along the rim, into the radiation path to make the radiation pulsed. Pulsed radiation is particularly useful because many types of infrared detectors only respond to changes in radiation level. For example, pyroelectric detectors, used in applications of photoacoustic spectroscopy and related techniques, require pulsed radiation. Pulsed radiation is also advantageous in electronic amplification and noise discrimination.
A non-mechanical means of providing pulsed radiation is disclosed by Nordal et al. in U.S. Pat. No. 4,620,104. In Nordal et al., thick film resistors mounted on ceramic substrates are electrically heated with pulsed current to generate pulsed infrared radiation without the use of mechanically moving parts.
Another means of modulating light is by using acousto-optical devices such as a Bragg cell. The operation of a Bragg cell is described briefly as follows. A Bragg cell is generally formed of a block of a crystalline material with a piezoelectric transducer bonded to an end or side of the block and is tuned to a frequency band suitable for the particular crystalline material of interest. The terms "piezoelectric transducer" and "transducer" will be used interchangeably hereinafter. When the transducer is excited with an electrical signal, a traveling acoustic wave is set up in the cell. This causes slight changes in the refractive index of the cell material between the peaks and valleys of the acoustic pressure wave. When light is introduced at the correct angle, termed the Bragg angle, the refractions from the index changes add in phase, and Bragg diffraction takes place. A portion of the input light beam is deflected and can be imaged onto a screen, photodetector or other device. The power of the deflected beam is proportional to the amplitude (power) of the acoustic input and the deflection angle is proportional to the frequency of the acoustic input.
The operation of a Bragg cell as an acousto-optic deflector or diffraction grating is described in greater detail as follows. A radio-frequency (RF) signal of center frequency .function..sub.c and bandwidth .DELTA..function. is applied to the piezoelectric transducer along a surface of the Bragg cell crystal. The RF signal causes the transducer to expand and contract with the RF signal frequencyf causing pressure waves to propagate down the width of the crystal. The wavelength of the acoustic pressure waves, .LAMBDA., equals v/.function., where v is the acoustic velocity (measured typically in mm/ .mu.s) of the crystal. The length of the crystal in the optical propagation direction is L. The width of the crystal, W, corresponds to the propagation distance of the acoustic waves and the width of the optical aperture. The height of the crystal, H, corresponds to the height of the optical aperture and sound field. The diameter of the optical input beam is D, where D.apprxeq.W.
For maximum efficiency, the input light angle relative to the acoustic wavefronts in the crystal is restricted by the Bragg condition: EQU K.sub.o =K.sub.i +K.sub.a, (5)
where K is the vector wavenumber with magnitude 2 .pi./.lambda. (or 2 .pi./.LAMBDA. for K.sub.a), and subscripts "o", "i" and "a" are indicative of "out", "in" and "acoustic", respectively. This means ##EQU2##
where .lambda. is the optical wavelength and .theta..sub.B,i is the input Bragg angle measured from the Bragg cell's normal.
Assuming the Bragg condition (EQ. 5) is met, the diffraction efficiency, .eta., is equal to: ##EQU3##
where P is the RF power, and M.sub.2 in the Bragg cell crystal material's modulation figure of merit in units of area per power. The diffraction efficiency, .eta., is maximum when: ##EQU4##
The diffracted optical power at each wavelength is proportional to the RF power, P, at the corresponding frequency. However, too much power will lower the efficiency due to the sin.sup.2 function. The absolute diffracted optical power is controlled primarily by the brightness of the source.
The diffraction (deflection) angle in air, .theta..sub.o, relative to the input angle in the Bragg condition, is equal to: ##EQU5##
This means that different frequencies, as well as different wavelengths, are deflected to different angles. The maximum deflection angle, .theta..sub.max, is determined by the wavelength, .lambda., transducer frequency bandwidth, .DELTA..function., and center frequency, .function..sub.c : ##EQU6##
The maximum number of resolvable angles, N, for ideal spots separated by .lambda./W, is equal to: ##EQU7##
For real systems, which must satisfy Rayleigh's criterion, EQ. (11) is expressed as: ##EQU8##
Thus, to satisfy Rayleigh's criterion, spots must be separated by angles of: ##EQU9##
since .DELTA..function./.function.=.DELTA..lambda./.lambda.. The average angular resolution, .delta..theta..sub.avg, equals .DELTA..theta./N. Thus, maximizing the resolution of an optical system with a Bragg cell by minimizing the spot separation requires a material with a slow acoustic velocity, a transducer with a large frequency bandwidth, and a large optical beam diameter (crystal width). From EQ. 8, the efficiency of the deflected light is proportional to L/H. Therefore, the efficiency increases with L while the resolution improves with larger .DELTA..function. and smaller v. Furthermore, the deflection angle increased with .lambda. as well as with large .function. and small v.
Bragg cells have been used in a number different applications, for example: spectrum analyzers, as in U.S. Pat. No. 4,633,170 to Burns; optical signal processors, as in U.S. Pat. No. 4,962,382 to Lee; active recognition systems, as in U.S. Pat. No. 5,717,401 to Sabet-Peyman et al.; chip rate detectors, as in U.S. Pat. No. 4,652,817 to Yarborough et al.; beam shifters, as in U.S. Pat. No. 4,443,066 to Freyre; and multi-channel fiber optic sensor systems, as in U.S. Pat. No. 4,649,529 to Avicola. However, in none of these references is a Bragg cell combined with a broadband light source to create a spectrum projector with arbitrarily time variable and spectrally variable output. Furthermore, none of the apparatuses disclosed in these references appear to be able to project arbitrary time-varying spectra, or to simulate the emission or absorption spectra of chemical and biological agents. Thus, there is a need in the art for a variable acousto-optic spectrum projector.